1. Field of the Invention
The present invention relates to a piezoelectric element to generate or receive surface acoustic waves. The present invention also relates to a SAW touch screen incorporating piezoelectric elements used as a wave generator or receiver.
2. Description of the Related Art
Touch screens or touch panels are used as data input units to enable users to interact with a computer system for e.g. factory automation equipment, office automation equipment or automatic measuring device. As known, a touch screen is a force-sensitive device designed to detect an area on the screen that the user touches with his or her finger, for example. The computer system performs required data processing based on the information of a location detected by the touch screen and other necessary data.
Recently, attention has been drawn to “SAW” touch screens in which the touched area is detected based on the surface acoustic wave (SAW). A typical SAW touch screen includes a transparent substrate that has a detection region and a marginal region surrounding the detection region. The marginal region is provided with a plurality of piezoelectric elements as a wave generator or wave receiver. Examples of SAW touch screens are disclosed in Japanese patent application laid-open Nos. H06-149459 and H10-55240.
A conventional piezoelectric element used as a wave generator or receiver is constituted by an interdigitated transducer (IDT) patterned on the marginal region for each element and a piezoelectric layer formed on the marginal region to cover the IDT. The IDT consists of a pair of comb-like conductors each having a prescribed number of parallel electrode fingers (conductive branches). The electrode fingers of one comb-like conductor are arranged alternately with and parallel to those of the other comb-like conductor. The piezoelectric layer is formed of a piezoelectric material, which exhibits the piezoelectric effect (generation of electric polarization as a result of the application of mechanical stress) and the reverse piezoelectric effect (production of a mechanical distortion as a result of the application of a voltage).
Upon application of an alternating voltage to the IDT of a piezoelectric element, an electric field is generated between adjacent electrode fingers. As a result, mechanical distortions occur in the piezoelectric layer due to the reverse piezoelectric effect, thereby producing elastic waves in the piezoelectric layer. In this process, the most strongly excited wave is a wave whose wavelength is equal to the pitch of the electrode fingers of the IDT. The produced elastic waves propagate along the surface of the substrate, to reach the wave-receiving piezoelectric elements. At these wave receivers, alternating electric fields are generated between the electrode fingers of the IDT due to the piezoelectric effect in the piezoelectric layer. Accordingly, an electromotive force is produced, and an alternating current is outputted from the IDT.
In operation of the SAW touch screen, the wave-generating piezoelectric elements produce surface acoustic waves. The surface acoustic waves propagate through the detection region of the substrate, to be received by particular piezoelectric elements serving as the wave receivers. When a finger is held in contact with an area in the detection region, the amplitude of the surface acoustic wave decreases as the wave passes through the contact point. The damping of the wave is detected and analyzed to locate the contact point in the detection region.
In a SAW touch screen, the electromechanical conversion rate of the piezoelectric elements should be as high as possible for attaining reduction of the driving voltage and for improving the detection accuracy. Specifically, with a high electromechanical conversion rate, each piezoelectric element can generate elastic waves efficiently in response to the applied voltage (when used as a wave generator), or can output an alternating current efficiently in response to the received elastic wave (when used as a wave receiver). In such a situation, the insertion loss between input and output signals for piezoelectric elements is small, whereby the reduction of driving voltage and the improvement of detection accuracy are realized.
In the conventional SAW touch screens, however, the piezoelectric elements do not have a sufficiently high electro-mechanical conversion rate for attaining a desired reduction of the driving voltage and improvement of the detection accuracy.